1. Field of the Invention
The present invention relates to an exhaust purification apparatus for purifying engine exhaust gas, and more particularly, to an exhaust purification apparatus having a particulate filter that traps particulates contained in exhaust gas and an ammonia selective reduction-type NOx catalyst that selectively reduces NOx contained in exhaust gas by using ammonia as a reducing agent.
2. Description of the Related Art
Exhaust gas that is emitted from an engine such as a diesel engine contains particulates, NOx (nitrogen oxide) and the like, which are air pollutants. In order to prevent the particulate emission into the atmosphere, a conventional exhaust purification apparatus traps the particulates contained in exhaust gas by using a particulate filter that is set in an exhaust path of an engine.
Also in order to deal with NOx, another conventional exhaust purification apparatus has an ammonia selective reduction-type NOx catalyst that is interposed in an exhaust path of an engine and purifies exhaust gas by selectively reducing NOx with ammonia as a reducing agent. This apparatus supplies urea-water into the exhaust gas existing in the upstream of the ammonia selective reduction-type NOx catalyst. The urea-water is hydrolyzed by exhaust gas heat to produce ammonia, and this ammonia is supplied to the ammonia selective reduction-type NOx catalyst. Some of the ammonia supplied to the ammonia selective reduction-type NOx catalyst is once adsorbed to the ammonia selective reduction-type NOx catalyst. The ammonia selective reduction-type NOx catalyst promotes denitrifying reaction between the ammonia and the NOx contained in the exhaust gas. In this manner, NOx reduction is carried out.
For example, Unexamined Japanese Patent Publication No. 2007-162487 (hereinafter, referred to as Document 1) proposes an exhaust purification apparatus that is constructed by combining a particulate filter and an ammonia selective reduction-type NOx catalyst for the purpose of efficient particulate trapping and NOx reduction. The exhaust purification apparatus described in Document 1 is formed of an upstream casing and a downstream casing that is disposed in the downstream of the upstream casing and communicates with the upstream casing through a communication path. A pre-stage oxidizing catalyst is placed in the upstream casing, and the particulate filter is set in the downstream of the pre-stage oxidizing catalyst. One of the functions of the pre-stage oxidizing catalyst is to produce NO2 by oxidizing NO contained in exhaust gas. The NO2 is used for the continuous regeneration of the particulate filter.
An ammonia selective reduction-type NOx catalyst is placed in the downstream casing, and a post-stage oxidizing catalyst is set in the downstream of the ammonia selective reduction-type NOx catalyst. One of the functions of the post-stage oxidizing catalyst is to remove from exhaust gas the ammonia that has flowed out of the ammonia selective reduction-type NOx catalyst.
Interposed in the communication path connecting the upstream and downstream casings is a urea-water injector that injects and supplies urea-water into exhaust gas existing in the communication path. The urea-water injected from the urea-water injector is hydrolyzed by exhaust gas heat, and ammonia is produced. This ammonia is supplied to the ammonia selective reduction-type NOx catalyst as a reducing agent.
When the exhaust purification apparatus is thus constructed, an injecting direction of the urea-water that is injected from the urea-water injector interposed in the communication path is virtually orthogonal to a flowing direction of the exhaust gas within the communication path where the urea-water injector is interposed. Amount of the exhaust gas flowing through the communication path fluctuates according to the driving condition of the engine. When the exhaust gas flow rate is relatively low, the urea-water reaches a point that is relatively far from the urea-water injector as viewed in an injecting direction of the urea-water. In contrast, when the exhaust gas flow rate is relatively high, once the urea-water reaches a point that is relatively close to the urea-water injector as viewed in the injecting direction of the urea-water, the exhaust gas causes the urea-water to flow downstream. In other words, a zone containing a rich urea-water spray moves depending upon an exhaust flow rate. It is then difficult to spray the urea-water so that a certain distribution of the urea-water spray is obtained in the exhaust gas all the time.
One idea for injecting the urea-water along an exhaust gas flow as well as possible is, for example, to fix the urea-water injector to the communication path in an inclined position with respect to an axis of the communication path. Nevertheless, the urea-water injector still has to be fixed onto the circumferential wall of the communication path in a position directed from the outside and the inside of the communication path. For this reason, the urea-water injecting direction from the urea-water injector still does not coincide with the exhaust-gas flowing direction within the communication path. Consequently, as with the exhaust purification apparatus of Document 1, when the amount of the exhaust gas flowing through the communication path fluctuates, the zone containing the rich urea-water spray moves depending upon the exhaust flow rate. It is therefore difficult to spray the urea-water so that a certain distribution of the urea-water spray is obtained in the exhaust gas all the time.
If the distribution of the urea-water spray is changed according to the fluctuation of the exhaust flow rate, the distribution of the ammonia produced from the urea-water is also changed according to the fluctuation of the exhaust flow rate. The fluctuation of the exhaust flow rate makes uneven the distribution of the ammonia in the ammonia selective reduction-type NOx catalyst. This eventually causes the problem that the ammonia selective reduction-type NOx catalyst is deteriorated in purification efficiency.